In the art of injecting a thermoplastics material from an injection molding machine into a single cavity mold, it is common to use a sprue bushing which extends from the mold cavity through a wall of the mold and which has a semispherical seat for receiving the nozzle of the injection molding machine. The standard sprue bushing includes a 2-inch diameter base portion from which extends a 1-inch diameter tip or nose portion and is usually press fitted into a counterbored hole which extends generally to the center of the mold cavity. A tapered hole extends axially within the bushing from the part spherical nozzle seat to the flat end surface of the nose portion which is located flush with the mold surface defining the cavity. After a part or article has been molded and cooled, the mold is opened and the article is ejected with a projecting sprue integrally attached to the article. The removing of the sprue from the article requires another operation, and frequently the portion of the article from where the sprue is removed, must be machined to provide a surface of satisfactory appearance.
A number of methods have been proposed or used to eliminate the secondary operations of removing the sprue from a molded article and of surface finishing the article. For example, an internal heater has been mounted within the sprue bushing so that the plastics material confined within the sprue bushing after the molding operation, remains in a molten condition and does not solidify with the plastics material within the mold cavity. When the mold is opened, only the small portion of the plastics material located molding operation, remains in a molten condition and does not solidify with the plastics material within the mold cavity. When the mold is opened, only the small portion of the plastics material located within the discharge orifice of the sprue bushing, remains attached to the part. While on some parts, it is not necessary to remove this small projection remaining on the molded plastic part, on other parts, it is necessary that this projection be machined, thus requiring a secondary operation.
In some forms of multiple cavity injection plastic molds, a series of valve members are located within the runner passage with one valve member for each mold cavity. Each valve member has a pointed tip portion which normally seats on a conical discharge orifice under the pressure exerted by a coil compression spring pressing on the opposite end of the valve member. When the plastics material within the runner passage reaches a predetermined pressure, the valve members shift to open positions allowing the plastics material to flow into the corresponding mold cavities. It has also been proposed to insert electrical heating elements within the nozzle members of a multiple cavity mold to help prevent the plastics material within the runner passages from solidifying with the molded part. Such nozzle members within the runner for a multiple cavity mold are shown in Canadian Pat. No. 658,976.